Fuel system conversion advise

[TheC5Surgeon]

Just bought my first square body. Its a 1976 C20 burb 454. Its a beautiful truck with extremely little body and frame rust. The previous owner did a tbi conversion. Long story short the truck does not like to idle, wants to bog down when given above 50% throttle, and seems to be running extremely rich. I would like to convert it back to a carbureted setup but am completely new to carbureted world. I have done some searches but most of the resources seem to be for conversion from carb to tbi. My questions to start would be: what parts needed to convert to carb system and what resources would be best to have a little guidance through the build? I attached some pics of the truck and fuel system.

 

[dusterdude]

I cant see the intake very well,if its a 4 barrel intake you can keep that,if not,buy an intake and a carb.fuel pump,you can keep what you have and buy a regulator to keep the psi down around 5 psi or buy a stock engine mounted pump and not worry about it.im assuming this setup has a computer and a computer controlled distributor.if it does,buy a 4 pin distributor for a 76 or so model truck,that will make life easier

 

[bucket]

Looks like an old Holley ProJection 2 conversion setup. I don't think they were known for great reliability and driveability.

It also looks like a standard HEI distributor with vacuum advance.

 

[75gmck25]

If they left the old ‘76 vintage 4 barrel manifold in place, and it’s still a standard HEI, converting back should be straightforward, but will still cost a bit.

- get fuel pressure down to about 5 psi (tbi runs at about 58) . It looks like they just added an external electric pump. You could try to regulate that pressure down, but pressure regulators aren’t cheap. I would reinstall a stock mechanical pump and lines, and just go back to stock setup. If your fuel tank has 3 ports on the sending unit, ensure you get a fuel pump with a return line. In hot weather you need the tank return to cool the fuel and prevent vapor lock.

- install a carburetor . Edelbrock AVS2 with electric choke gets high marks for simple tuning. You may need to use a square bore to spread bore adapter if you still have a stock Quadrajet spread-bore intake. Quadrajets also work well, but quality and condition is unpredictable for old carburetors. Quadrajet is also more complicated to tune than the AVS2.
Holley carbs also work fine, and some really love how you can tweak them for max performance; however, it sounds like you just want it a little simpler.

- verify operation of the HEI. Set base timing to about 10-12 BTDC, and verify you get 18-20 more from mechanical by about 2500 rpm before connecting the vacuum advance. Connect vacuum advance to full vacuum on the carburetor and look for 15- 20 degrees vacuum advance.

- for a big block you might need a tuning kit or one of the higher cfm carburetors to get best power. Most of the off-the-shelf carburetors are about 600-650 cfm, and are tuned for economy on a 350 cu in engine.

 

[TheC5Surgeon]

Thanks for the replies guys. So of the two main choices I could either keep the line with the electric fuel pump and just add a regulator in series correct? Or I could replace that electric fuel pump and line with stock line and run a mechanical fuel pump and extra line going from pump to carb?
To me it would make more sense to run a mechanical setup but it does seem simpler to just add a regulator in series if that's all that would be needed. Also how can I be sure that the stock camshaft is in the motor that has the mechanical fuel pump provision?

 

[Raider L]

@TheC5Surgeon,

Guys does that look like the TV cable is disconnected? Surgeon, I didn't even think about that fuel pump lobe on the cam not being there. But whoever did the conversion probably just put a block off plate on the engine. I would really check it out closely to make sure you could even get a regulator for pressures that high that would turn down that low, 5-6 psi! Some regulators that go down that low won't go up that high. They aren't all universal, they are specific for a certain range of pressures low to high. That's why I would ask about the range of any regulator you were looking at.
Or take that thing, the pump, off and get you a good brand mechanical fuel pump. Even the factory fuel pump for the 454 would be okay if you're just doing this for a daily driver, which I would suppose you are. It would be well in the correct range and would be cheaper than racing after market pumps. And easier to install.

 

[TheC5Surgeon]

So I am starting to order parts for this conversion. Just ordered main fuel (3/8 inch) and return (1/4 inch) pre-bent stock replacement lines from inline tube. One thing I am wondering about is what connects to the fuel pump ports. The replacement mechanical fuel pumps I can find (https://shop.advanceautoparts.com/p/carquest-premium-mechanical-fuel-pump-260573/11763962-P) show three ports on the pump. I can assume one is for the the main fuel line, one for the return. I'm guessing the other goes from the pump to carb correct? The replacement pump to carb lines seem to be male fittings (https://www.inlinetube.com/products/ctc7305). Would the main and return lines need rubber hoses connecting them to the pump? Thanks

Matthew

 

[75gmck25]

When mounted on the engine, the threaded fitting for the metal line to the carburetor is closest to the engine. The next one is the small return line and the one closest to the frame rail is the 3/8” fuel feed line. The return and feed lines have short rubber hoses connecting them to the metal lines inside the frame rail.

 

[eric 87]

So I am starting to order parts for this conversion. Just ordered main fuel (3/8 inch) and return (1/4 inch) pre-bent stock replacement lines from inline tube. One thing I am wondering about is what connects to the fuel pump ports. The replacement mechanical fuel pumps I can find (https://shop.advanceautoparts.com/p/carquest-premium-mechanical-fuel-pump-260573/11763962-P) show three ports on the pump. I can assume one is for the the main fuel line, one for the return. I'm guessing the other goes from the pump to carb correct? The replacement pump to carb lines seem to be male fittings (https://www.inlinetube.com/products/ctc7305). Would the main and return lines need rubber hoses connecting them to the pump? Thanks

Matthew

I am probably really late to this conversation. I have been researching this myself and could help you with the parts list. And mine is an 87 Sierra 305 TBI. I may soon be switching to carb myself but mine did come with the TBI. If your block and heads are stock then the intake isn't an issue. Mine has the TBI heads and the bolt angle for the center bolts is different. The fuel pressure for my TBI system is only about 13 so I am not sure why other say yours is so high. I would check that since you have no idea what the other guy did. The TBI's need that kind of pressure and it may actually be your problem if he didn't have the correct pressure. It would starve and probably make the computer try to adjust and be rich running.
Holley makes a cheap enough regulator to drop pressures back down to 6 which is fine for carbs. It can mount on firewall. I would check the pressure first.
But a 600cfm is plenty carb no matter the make for a stock or near stock engine. Curious about the distributor as well. TBI's typically use a computer controlled distributor so you may have to replace it. If you use my parts prices I think you will be around 800-900 if you buy all new parts. Does your tanks have the built in fuel pump/s. One tank or two?
Since it is a burb I remember one tank so to be safe just get a mechanical if you have the original block or a low pressure I think Holley's red line electric is low enough. A 600cfm holley or edelbrock or one of the new holleys like the avs2 or street warriors. A distributor if the current one is computer controlled to a vacuum advanced model HEI. The intake might be useable so you save there and will spend less than I am if I do this conversion.
NOW others may have better info since I am comparing a shopping list for my 87 which was the first year of computer controlled TBI and Distributor. And I hope to get it straightend out but if not then I may bypass the conversion for a carbed replacement engine from Jegs. A dressed one.

 

[TheC5Surgeon]

Just an update. I ended up not even using the new fuel line I ordered from inline tube. Instead I just took out the electronic fuel pump and put rubber fuel hose in its place. I got the edelbrock carb, new mechanical fuel pump , filter , drained the tank , and the truck fired right up without hesitation. I took it on a little test drive and the truck ran perfect until it got fully warmed it started to try and die at idle. I still need to tune the carb and verify/set timing. Unfortunately the truck is in Illinois in my in-laws barn so I dont have current access.

 

[Bextreme04]

Just an update. I ended up not even using the new fuel line I ordered from inline tube. Instead I just took out the electronic fuel pump and put rubber fuel hose in its place. I got the edelbrock carb, new mechanical fuel pump , filter , drained the tank , and the truck fired right up without hesitation. I took it on a little test drive and the truck ran perfect until it got fully warmed it started to try and die at idle. I still need to tune the carb and verify/set timing. Unfortunately the truck is in Illinois in my in-laws barn so I dont have current access.

Probably just need to set the ignition, idle setting, and idle mixture when warm. The numbers given above for timing are a bit high for a 454. I'd suggest start with less that that, maybe 8 degrees at idle with vacuum advance disconnected. Then set the idle mixture screws. Then set the idle rpm with the idle adjustment screw after you reconnect the vacuum advance. Once all of that is done, you WILL need to reset the high idle on the next cold start. Just give the throttle a single pump to the floor with a slow release to set the choke and get the throttle plate on the high idle cam. Then crank until it starts. Once it is started, adjust the high idle screw(different screw than the regular idle) to get 900-1200rpm. I usually shoot for 900 at startup and it will climb to 1200 as it warms and the choke pulls off. You should then be able to tap the accelerator pedal and have the idle drop back down to ~650rpm.

 

[TheC5Surgeon]

Thanks for the info Bextreme. Should be going back to IL to fine tune it by late spring